Relationships between road accidents and hourly traffic flow—II: Probabilistic approach

This is a continuation of the investigation into relationships between accident rate and hourly traffic flow as outlined in Part I of the research. The underlying study attempts to determine appropriate models for single- and multi-vehicle accident rates in conjunction with free-flow and congested-flow conditions. For the free-flow data, the total accident rate-hourly flow curve follows the U-shaped configuration. This form is the result of a convex downward and a convex upward curve for single- and multi-vehicle accidents, respectively. For the congested-flow data (characterized by multi-vehicle accidents), the accident rate increases sharply with hourly flow. The models are applied to probabilistic aspects with consideration of a fitted, generalized (hourly flow dependent) headway model. The headway model represents the probability that two vehicles which are, even instantaneously, under a car-following mode are in a potentially hazardous situation. The approach investigated is believed to provide an essential input for both simulation studies and theoretical models of road traffic accidents.